1. Field of the Invention
This invention relates to the field of torque converters and fluid couplings for use in automatic transmissions for motor vehicles.
2. Description of the Prior Art
Conventionally, the turbine of a hydrokinetic torque converter comprises three components: a shell, vanes, and a shroud. In order to structurally join these three components, both the shell and shroud are currently slotted to receive, through the slots, tabs formed on the vanes. After the vanes are located within the shroud, the tabs are bent or rolled over to form a mechanical attachment on the shroud that holds the vanes fixed in position. To facilitate automatic vane assembly, clearance must be provided between the slots and the tabs. This clearance results in leakage of hydraulic fluid through the thickness of the shell, which leakage causes a substantial penalty performance, operating efficiency and fuel economy. A turbine wheel formed of fiber-reinforced plastic resin composite material eliminates the need for slots through the shell thickness because the shell, vanes, and shroud are molded integrally. Furthermore, a composite turbine wheel also eliminates the gap between the shell and vanes and offers potential for significant improvement in fuel economy. An additional advantage resides in the incorporation of an airfoil-shaped blade. U.S. Pat. No. 5,226,807 describes a turbine wheel of this type.
Loads carried by the turbine wheel are transmitted to an input shaft of the transmission through a spline connection. Use of a spline for this purpose requires that the hub of the turbine wheel be formed of metal and that it be part of the load path between the turbine and transmission input shaft. The turbine is subjected to axially directed loads, torsion, and radially directed centrifugal forces and thermally induced loads.
A rigid joint between a plastic turbine wheel and a metallic hub can cause significant difficulties. For example, the composite plastic material has a lower stiffness and strength in comparison to the strength and stiffness of the metallic hub. If the hub and turbine wheel are attached mutually by a rigid connection, they are required to deflect as a single structural member. Such a rigid attachment can induce high stresses into the composite material.
Another difficulty is associated with the substantially greater thermal expansion coefficient of the composite turbine wheel in comparison to that of a metal hub. Because of this, temperature within an operating torque converter, which may vary between -40.degree. F. and 450.degree. F., can cause large thermal stresses in the composite material.